Add a macro for print in debug mode only

[3442853561]

I not sure is RFC #2173 include this issue, or not.

macro_rules! debug {
    ($($arg:tt)*) => { 
        if cfg!(debug_assertions) {
            print!($($arg)*);
        }
    };
}

fn main() {
    debug!("test");
}

[Centril]

It does (and more), but to STDERR instead of STDOUT. Read the RFC on a motivation of why.

[Centril]

I'll go ahead and close this one since I think it is sufficiently close to dbg! (latest RFC: #2361).